Extensive fishing of natural waters has lead to a reduction in fish numbers. It is now recognized that fishing at a rate to sustain natural populations will not provide the world's needs for fish as a food. This has lead to the development of the aquaculture industry, in which fish and other aquatic species are produced in controlled bodies of water. Fish is, worldwide, the single biggest source of protein, and aquaculture is therefore an increasingly important means of producing food. Furthermore, since the fish are in a controlled body of water, means are being sought to control disease and maximize production. The present invention provides a new technique for improved fish production.